#include "iomanip"
#include "fstream"

int main()
{
	std::ifstream input("in.txt");
	std::ofstream output("out.txt");
	double x, y;
	double answer = 0;
	input >> x >> y;
	double x1, x2, y1, y2;
	input >> x1 >> y1;
	while (input >> x2 >> y2)
	{
		answer = answer + ((x1 - x) * (y2 - y) - (x2 - x) * (y1 - y)) / 2;
		x1 = x2;
		y1 = y2;
	}
	output << std::setprecision(10) << std::abs(answer) << '\n';
	return 0;
}